Displaying the Future

Embedded Displays Go Light and Cool

by Jim Turley

To see the future of embedded design, all we have to do is watch Tom Cruise.

Partway through “Minority Report,” the actor waves his hands in front on an enormous projection screen/hologram. He’s pushing around icons and shifting windows, an action we recognize immediately because they’re just like what we do every day with a typical Mac, PC, or iPhone. The only thing that’s different, really, is that he’s not physically touching anything. It’s all very cool and frankly is the only part of the movie most of us remember.

Elsewhere in the movie, he opens a cereal box and studiously ignores the TV commercial that’s somehow playing on the back of the box. As interesting and unexpected as these effects are, neither technology is out of reach of today’s embedded designers. In fact, the display-on-a-cardboard-box stuff is pretty widely available and could be very handy for small, curved, space-constrained, or power-limited designs.

A half-dozen companies produce seven-segment LCD displays that are thin enough (about 0.5 mm thick) to embed right into your own cardboard box or the face plate of small embedded systems. Originally developed for “smart” credit cards, these displays are also moderately flexible and use almost no power. They’re not fast – the displays sometimes take nearly a second to update – but for gee-whiz factor they’re hard to beat.

Aveso, SiPix, eInk, Citala, Kent Displays, and other companies all produce similar modules that are competitive with one another. They’re all flat, thin, and low-power, and they all have seven-segment (i.e., numeric) displays like you’d see on a calculator. Their interfaces are different, and some modules have more intelligence than others. Some vendors even give you a choice of colors, if that’s important to you.

The Primero module from Aveso is typical of most companies’ offerings. It’s got a six-digit display laminated to a flexible printed circuit that contains the display drivers and a Texas Instruments MSP430 controller chip. The display itself is about an inch high and appears blue on a golden background. The MSP430 chip is there to handle the intricacies of the display. All you have to do is talk to the microcontroller and let it update the display. Not incidentally, the MSP430 is overkill for this application; it’s got plenty of excess performance and memory space for your own software. As long as you’re happy writing MSP430 code, it’s probably the only microcontroller you’ll need for a reasonable display subsystem.

If you dislike TI’s MSP430 or you just prefer to “bit bang” your own display drivers, you can also buy the module without the microcontroller. It’s obviously cheaper, but you’ll have to babysit each display segment to keep them updated and refreshed. In this “headless” mode you’ll have nearly 100 I/O lines to manage. Still, if you’ve already got a microprocessor or microcontroller in your system and just want to add a flat, dumb display this may be just the ticket.

SiPix offers a similar product, which it calls ePaper. Like Aveso’s display, it’s about an inch high, a fraction of a millimeter thick, and reasonably flexible. It’s also electrophoretic (it doesn’t glow, it’s reflective) and bistable, meaning the display holds its image even after you’ve removed the power. The effect is kind of eerie. You can literally yank the power leads off the display and it doesn’t go blank. Very weird but also very useful for saving energy. SiPix offers more color choices than Aveso, but, like Aveso, SiPix sells the display either with or without an embedded microcontroller. In the case of SiPix, it’s a 6502-based chip, so you’ll finally be able to put those Apple ][ programming skills you learned in high school to good use.

Compared to the first two companies, eInk has a much wider choice of products than either Aveso or SiPix. This is the company that makes the display used in Amazon’s Kindle electronic book reader as well as similar products from other companies. At eInk, the company focus is really on larger pixel-addressable displays, with the thin digital displays something of a sideline. Even so, the product is good, and if you’re contemplating an upgrade to a larger display or one that can handle graphics, eInk has got you covered. The eInk display is black on white (it’s meant to look like paper) and bistable, so it uses no power whatsoever when it’s not being updated. Because of eInk’s manufacturing process, its displays aren’t as thin as some of the others; eInk’s full-size displays have glass substrates so they’re obviously too thick for the futuristic cereal box but perfectly adequate for more mundane applications.

All of these companies are quite happy to develop custom displays for you if you’re not satisfied with the standard seven-segment numeric versions. Given the right volume commitments and/or NRE charges, they’ll gladly create a display with your own custom icons, graphics, logos, or glyphs. Citala, in particular, prides itself on its customization capabilities. Although the company does produce standard eight-digit numeric displays similar to the others’, it thrives on custom display work. For a medical or automotive application, or even just a high-volume consumer item, a custom zero-power display could make a nice addition to the design. One maker of flash drives puts a bar graph “fuel gauge” on the side to indicate the remaining capacity of the drive.

Kent Displays is a bit different from the other companies in that it doesn’t actually manufacture anything. Instead, the company is a research spinoff that’s developed the underlying technology behind its Reflex display for others to manufacture. As such, you can’t buy displays directly from Kent, but they’ll be happy to direct you to their licensees.

One advantage all these displays share is that they’re sunlight readable. Since they’re all reflective (as opposed to glowing like LEDs), they actually look better outdoors than inside. On the other hand, that can be a disadvantage in dark environments where there’s little ambient light. The necessary backlight would defeat the power-saving advantages of the reflective display. They’re also all bistable, so power consumption drops to exactly zero when the display isn’t changing. That’s great for an e-book reader, where the display changes once every few minutes, but it’s irrelevant for a lab instrument or consumer item where the display might change constantly. Still, power savings is never a bad thing, even if you don’t take advantage of it very often.

Likewise, these displays’ flexibility is either a fantastic boon or irrelevant feature. If you want to wrap a simple numeric display around a curved surface, there aren’t many alternatives. If your system is flat and boxy, however, that flexibility may not buy you much apart from reduced fragility (these units won’t shatter like glass-backed displays). In “2001: A Space Odyssey” astronaut Dave Bowman casually rolls up a thin and flexible TV screen. Although it took a few more years than Stanley Kubrick might have imagined, the movie was a pretty good predictor of what was to come. Even if it didn’t include Tom Cruise.

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